1. Field of the Invention
This invention relates to the field of an improved method for hardening the surface of steel and cast iron, and, more specifically, to a carbonitriding process for iron-containing articles.
2. Prior Art
Surface hardening methods which make use of the usual nitriding processes have in the past depended upon ammonia gas or molten salt bath nitriding. Of these, the ammonia gas nitriding method has the severe drawback of lengthened treating times and producing a brittle layer on the article. As for the salt bath nitriding method, a surface layer is also produced which needs after-treatment. In addition, the salt bath used is highly toxic.
As the term "carbonitriding" implies, this known process is a combination of carburizing and nitriding. Carburizing involves the application of carbon to a ferrous metal article with the object of increasing the carbon content of the surface and in some cases the nitrogen content as well. This is done such that when the article is subjected to a suitable heat treatment, the surface portion thus carburizes, resulting in a substantially harder surface than the underlying metal. This has been known in the art as "case hardening". Prior art methods of carburizing have been accomplished by a variety of different methods such as, for example, pack carburizing, gas carburizing, carbonitriding and liquid carburizing. The mechanism of the carburization of steel involves a diffusion of the carbon into the steel itself. Thus, the theory behind this process is related to the law of diffusion.
Pack carburizing referred to above involves a method wherein carbon is formed on the surface of the steel by the decomposition of carbon monoxide into carbon and carbon dioxide. The carbon dioxide that is formed reacts immediately with the incondensed carbon in the compound to produce fresh carbon monoxide. This process is repeated as long as there is enough carbon present to react with the excess of carbon dioxide. The decomposition of carbon monoxide occurs at the surface of the steel which must therefore be at a temperature such that the carbon form will dissolve into the steel. The usual commercially used carburizing compounds contain a mixture of approximately 20% alkali or other metal carbonates burned to a hardwood charcoal by the use of oil, tar and the like. In the operation of this method, the pieces to be carburized are packed into a box and a layer of the compounds is disposed about them. As very high temperatures are used, the boxes used to contain the articles to be carburized are relatively expensive and are usually made of a nickel-chromium alloy. The articles are then heated to a carburizing temperature of between 675.degree. and 700.degree. C. where the above reaction takes place.
In gas carburizing, the carbon of the furnace atmosphere is in the form of gaseous hydrocarbon compounds or carbon monoxide. In order to transfer the carbon to the steel, the gaseous compound must react with the steel on the surface or in the immediate subsurface of the article. Usually, a reaction wherein methane gas is broken down into its various components is used in gas carburizing.
Finally, liquid carburizing is a method in which the steel or iron is placed in a molten salt bath that contains the chemicals required to produce a chafe comparable with one resulting from pack or gas carburizing. Carburizing in liquid baths provides a method for case hardening with load distortion but requires the use of difficult-to-work-with materials such as barium cyanide and the like. The carburizing bath usually operates at a temperature of about 1600.degree. F. and the piece is placed in the bath for a predetermined length of time such that the carbon diffuses into the surface of the metal.
On the other hand, cyaniding is substantially the same as carbonizing but, in this case, CN is what is being diffused into the surface. Cyaniding is known in the art to produce a hard, superficial wearing surface on various steel articles. Steels treated by this method absorb both carbon and nitrogen from, for example, a molten salt bath, and when quenched in water or in suitable oil, they develop the aforementioned hardness in their outer surface. In the prior art, sodium cyanide in the form of a bath is widely used. However, cyanides are known to be violent poisons and therefore such process is dangerous and relatively expensive to carry out. Baths of the sodium cyanide are employed as the heating or reheating medium in connection with the hardening of steels and act as a liquid medium for case carburizing affording a quick means of obtaining a hard superficial wearing surface. Ordinarily, cyaniding is carried out at a temperature just higher than the upper transformation point of the core of the article and the steel is generally quenched directly from the cyaniding temperature. This temperature in the case, which does not approach a tool steel composition and which has not been held for a sufficient length of time to cause brittleness from excessive grain growth, or quenched to the desired hardness in the core, will be thoroughly refined and tough.
Nitriding is a process which consists of subjecting the steel to the action of a nitrogen medium, generally ammonia gas, under conditions whereby high surface hardness is imparted to the steel without necessitating any further treatment. Nitriding is generally accomplished utilizing a relatively low temperature in which the parts are hardened and does not require quenching after exposure to the ammonia gas. Under this process, the parts are subjected to the ammonia gas at a high temperature which produces nitrogen, which is very active at the moment of the decomposition of the gas, provides to a certain extent the alloying elements of the steel to form nitrides. These nitrides, in a fine state of dispersion in the case, impart extreme hardness to the surface of the steel, a hardness that generally decreases in (relief) until it corresponds to that of the core.
Combining these two methods of carburizing and nitriding has led to the use of a method called carbonitriding or gas cyaniding. This is a process for case hardening a steel part in a gas carburizing atmosphere which contains ammonia gas in controlled percentages. Both carbon and nitrogen are additive steel; the nitrogen serves chiefly to reduce the critical cooling rate of the case and the carbon content of the gas is controlled as hereinbefore described. The nitrogen content of the surfaces is controlled by maintaining the desired ammonia content in the furnace atmosphere through varying the amount of the ammonia added.
The nitrogen and carbon contents of a steel part after carbonitriding at a given temperature decrease from the surface to the core, and since both the nitrogen and carbon contents have a pronounced effect on the critical cooling rate, the depth of the case must always be conditioned in terms of the depth of the effective or hard case. Carbonitriding is no different in this respect from the other methods mentioned hereinabove for producing carbon-nitrogen case.
Thus, the prior art methods, while they may be adequate to produce a good case-hardened article, are either expensive or require the use of exacting techniques and dangerous chemical process steps. The present invention represents an advancement in the art of carbonitriding and contains none of the aforementioned shortcomings associated with prior art production methods. The present invention utilizes an after-burner technique for any HCN which should be produced in the instant method. Thus, even if the present method should produce a "noxious" gas, it is effectively burned and therefore renders the instant method relatively safe to use. Inasmuch as the only gases which need be used in order to carbonitrize the metal are relatively inexpensive ammonia gas and well-known organic solvents, the expense associated with the initial method is substantially reduced especially when compared with that of maintaining metal salt baths and the like.